In recent years the mining industry has become increasingly involved with the development of methods for extracting gold from complex refractory ores. This trend is due to the increasing scarcity of free milling gold ore deposits. Gold ores are considered refractory when their gold content cannot be recovered by standard cyanidation techniques. Such ores are often refractory because of their excessive content of metallic sulfides or organic carbonaceous matter or both. It is not clearly understood why the sulfides and carbonaceous material cause such gold ores to be refractory, but the phenomenon is well known in the mining industry.
The treatment of refractory gold ores is the object of several recently developed processes. U.S. Pat. No. 4,289,532 to Matson et al treats carbonaceous ores with oxygen and chlorine prior to a cyanidation step. Matson's oxygenation step is carried out at atmospheric pressure and slightly elevated temperatures, i.e. 120.degree.-210.degree. F., and is followed by a chlorination step which is also conducted at atmospheric pressure and at a temperature of 70.degree. to 140.degree. F. Matson's oxygenation typically requires between 4 and 20 hours and preferably about 8 hours. In Matson's examples, chlorination is typically conducted with a retention time of 4 to 6 hours. The commercial implementation of this technology necessitates the utilization of large equipment in the oxygenation and chlorination sections of the gold mill to provide for the required retention times.
Other U.S. patents involving oxygenation and/or chlorination suffer from similar retention time requirements. For example, U.S. Pat. No. 3,846,124 utilizes a pretreatment using a chlorination step extending over a period of about 24 hours. U.S. Pat. No. 4,188,208 utilizes oxygenation for a period of about 24 hours. U.S. Pat. No. 4,188,208 utilizes oxygenation for a period of 8 hours prior to cyanidation of the gold ore. U.S. Pat. No. 4,038,362 discloses a process for the pretreatment of refractory gold ores in which an aqueous slurry of the ore is first oxidized with air over a period of 8 to 24 hours and then with chlorine for another 8 hours.
While all of the processes claimed in the foregoing patents operate at atmospheric pressures, other processes have been developed which operate at elevated oxygen pressures. For example U.S. Pat. Nos. 4,552,589 and 4,738,718 disclose processes in which refractory gold ores are subjected to elevated oxygen pressures in autoclaves. Even in the more extreme ranges of temperature (400.degree.-500.degree. F.) and oxygen overpressure (50-100 psig) cited in the '718 patent, autoclave retention times of about 4 hours are required to obtain gold extractions of about 90%. These processes also suffer from the further disadvantage that both require preheating of the ore to above 300.degree. F.
In light of the foregoing, a need exists for a method of treating refractory gold ores wherein the retention time of the treatment step is minimized and the ore slurry does not require heating prior to the treatment.
Accordingly, it is an object of this invention to provide a method for treating refractory gold ores to render such ores amenable to conventional cyanidation technology, said method requiring minimal retention time and minimal heat input.
Furthermore, it is an object of the present invention to provide an economically advantageous method for treating refractory gold ores to make them amenable to conventional gold leaching techniques.
These and other objects of the invention will become apparent from the description of the invention.